A previously undisclosed sound recording captured the shooting of Michael Brown by a white police officer, CNN reported Tuesday, as it broadcast what it said appeared to be audio from the gunfire that killed the unarmed black teen.

The FBI, contacted today by AFP, refused to comment on the authenticity of the recording, reportedly made by a resident of Ferguson, the Missouri town where the fatal shooting took place.

The man was said to have been recording a Skype conversation at the same time as the deadly encounter nearby between Brown and the police officer, CNN said.

Six gunshots are heard on the brief recording broadcast by CNN, followed by a brief pause, then four more shots.

The man who made the recording wishes to remain anonymous, said his lawyer Lopa Blumenthal, who said it could have "huge relevance" in the case -- particularly the pause in the shooting.

"It's not just the number of gunshots, it's how they're fired," Blumenthal said.

"That has a huge relevance on how this case might finally end up."

A grand jury is hearing evidence to determine whether police officer Darren Wilson, 28, used excessive force in fatally shooing Brown, who autopsies found had been shot six times.

Accounts of the shooting differ greatly, with police alleging Brown was trying to grab Wilson's gun.

But witnesses, including a friend walking with Brown, said he was shot as he held his hands in the air in a clear sign of surrender.

The teen's August 9 shooting sparked days of protests and spasms of violence in Ferguson, a suburb of St. Louis, Missouri. It also triggered a national debate on race across the United States.

The teen was laid to rest yesterday at a funeral attended by thousands, including US civil rights leaders Jesse Jackson and Al Sharpton, as well as representatives dispatched to Ferguson by US President Barack Obama.